The invention relates to a charge amplifier circuit, the output of which is fed back to the signal input through a drift-compensating device, the drift-compensating device being connected to a trigger device via which a trigger signal can be supplied. When a trigger signal occurs, a compensating current corresponding to the drift of the signal output by the charge amplifier, is generated at the output of the drift-compensating device and applied to the signal input of the charge amplifier.
With charge amplifiers employed at the present day, for example, in connection with piezo-electric measuring systems, it is necessary either to use input stages with the highest possible isolation which give rise only to negligible input leakage currents, so that these input stages do not cause any disturbing drift of the output voltage of the charge amplifiers, or to balance out, by appropriate compensation, any disturbing drifting of the output voltage that may occur, to thereby eliminate the influence of the drifting on an amplified measure signal produced by the charge amplifier.
A circuit of the kind stated in the introduction and which operates on the basis of the second-mentioned principle is known, for example, from Austrian patent AT-PS 377.132. In the disclosed circuit of AT-PS 377.132, to compensate for the drift, there is provided at the output of the charge amplifier a circuit arrangement with a DC amplifier and an analog-digital converter, followed by a digital-analog converter and a resistor, through which, prior to the measurement, the input leakage currents are compensated automatically in a balancing phase by a compensating current, this current being held constant during the subsequent measuring phase. After actuation of the trigger device, in this known circuit, the most recently started digitizing process is halted and then the input of the charge amplifier is acted on by the compensating current corresponding to the last-determined drift, as long as the system is in the "measuring" mode.
This known circuit is suited for carrying out individual measurements, for example, such as those that arise in ballistics in connection with determining the pressure in the barrel of a weapon being fired. But, this circuit has only limited suitability for evaluating periodically occurring measured signals, especially when these periods are very short, such as, for example, in the evaluation of pressure measurements on high speed internal combustion engines. This is because, on the one hand, the forms of A/D converters and D/A converters capable of use for this purpose are relatively complicated and expensive and, on the other hand, in the known circuit there are no measurements performed or initiated for easily and conveniently introducing a periodic compensation in the intervals between the individually arising measured signals.
Regarding the last-mentioned deficiency, there is known an arrangement, for example, from West German laid open application DE-OS No. 36 32 221, which includes a circuit that is substantially distinguishable from that cited in the introduction, wherein, to stabilize the zero point of the output of the charge amplifier, the output signal is measured at a predetermined instant at which the desired voltage or desired measured value is known, and then is suddenly corrected to this value by a type of reset arrangement. The outlay of hardware rises as a consequence of the need to provide for this device individual trigger signals characterizing the periodic measuring signals and continuously running clock signals or angle marker signals. Further, because of the periodic resetting of the output of the charge amplifier, no significant measured signal can be relied upon for all variations of drifting.